1. Field of the Invention
The present invention relates to a technology for bonding a flexible printed-circuit board to another printed circuit board.
2. Description of the Related Art
Recently, there is an increasing demand for reducing a size and improving a design flexibility of small electronic devices such as cellular telephones. To satisfy such a demand, flexible printed-circuit (FPC) boards have been widely used, in addition to the conventional electronic printed-circuit boards made of an electric insulating material such as glass epoxy that has been used for a long time. In the FPC board, a printed circuit board, which is a wiring pattern, is provided on the surface of or inside a flexible resin film.
When the electronic printed-circuit board and the FPC board are used together, electronic parts and/or wiring of the electronic printed-circuit board needs to be electrically connected to the FPC. If the circuit boards are designed for a relatively large electronic device, the device has enough space to fix the electronic printed-circuit board and the FPC board by using, i.e., fittings, and to connect the electronic printed-circuit board and the FPC board through, i.e., lead wires.
On the other hand, because a small electronic device cannot afford such space required for the fixation and the connection, the electronic printed-circuit board and the FPC board are often bonded using, i.e., an adhesive, and connected together by performing wire-bonding between the electronic parts of the electronic printed-circuit board and the FPC board. For example, Japanese Patent Application Laid-open No. H6-275931 discloses a technology for improving productivity that is otherwise to be lowered due to a contact failure and realizing a reliable wire-bonding, by adjusting the thickness of the material of the FPC board when the wire-bonding is performed between the electronic parts and the FPC board.
However, according to the disclosed technologies, problems caused by the adhesive are not considered. To perform the wire-bonding, an ultrasonic wire-bonding method is generally used, with which wires and an object is bonded by applying ultrasonic vibration to the wires made of, i.e., gold, and using friction generated between the wires and the object to be connected to the wires. The contact effect can be improved by applying heat to the object to be bonded to the wires.
Thus, when the wire-bonding is performed on the FPC board adhered to the electronic printed-circuit board, it is important to sufficiently apply ultrasound waves and heat to a contact portion. However, if the adhesive layer that bonds the electronic printed-circuit board and the FPC board is relatively thick, the layer absorbs the ultrasonic vibration and prevents the heat from properly flowing to the contact portion, which causes a connection failure. On the other hand, if the adhesive layer is relatively thin, the fixation strength is weakened, which increases the risk of the FPC board coming off from the electronic printed-circuit board.